Conventionally, there are circuits that correct, when the amount of change in phase compensation exceeds a threshold and thus the generation of a slip is detected, phase change due to the slip by adding a phase correction amount obtained from the amount of change in phase compensation to the phase of a signal the phase deviation of which has been compensated. For example, such a circuit is disclosed in International Publication Pamphlet No. WO 2012/132103. There are methods in which a known signal, such as a known bit or code, is embedded between a synchronizing burst that is periodically inserted in an optical signal and a consecutive synchronizing burst and detection and correction of a cycle slip by the known signal is performed at a receiving side. For example, such a method is disclosed in International Publication Pamphlet No. WO 2010/063092. There are devices that detect the generation of a phase slip by inserting a known signal in a transmission data stream at certain intervals at a transmission side, obtaining a known signal from a reception data stream at a receiving side, and calculating a relative phase difference between the known signals. For example, such a device is disclosed in Japanese Laid-open Patent Publication No. 2007-053571.